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Title :  Expansion Joints: Types & Selection

 1.0 BELLOWS CONVOLUTIONS
The bellows convolution is the element of the bellows that makes it flexible. Each convolution is like a pair of thin washers, which can deflect easily by bending parallel to the bellows centerline. The amount of flexibility of each of these convolution is a direct function of the height of the convolution. The height is limited by both the pressure and the ability to form the convolution. Except in a very few applications, a single convolution is not capable of providing a sufficient amount of movement.

The convolution can only deflect in an axial direction in any radial plane. Therefore, a single convolution can only accept movement in an axial direction, or angularly about its center. A single convolution is incapable of accepting lateral deflection. Therefore, the bellows usually will have several convolutions. At certain calculated critical pressure each convolution will experience a deformation of the sidewalls known as in-plane instability. Every bellows also has a calculable critical pressure at which column instability will occur. The column buckling, which is also known as squirm, occurs to convolutions as a whole, rather than individually.

Each bellows are computer designed and analyzed to assure that the required pressure and movement considerations and limitations have been taken into account.

Single Expansion Joints have one bellows. Axial compression and extension, lateral and angular movement can be accommodated. These expansion joints cannot restrain the internal pressure thrust. The piping designer must provide the system with separate anchoring to resist the pressure thrust.

Tied Single Expansion Joints also have one bellows, except that their length is restrained by tie rods, which can accept the pressure generated thrust. When under pressure they are usually not designed to accept any axial movement, but can accept lateral movement. The two tie rod designs can accept angular deflection in a single plane.

Hinged expansion Joints have a single bellows and have their length restrained by the addition of a hinge mechanism which is strong enough to accept the pressure generated thrust but will allow angular movement in a single plane.

Slotted Hinge Expansion Joints are a variant of the above “hinged” type, in that while providing for angular movements, some axial movement can be accepted because the hinge pin passes through slots in the hinge plates. When the axial movement is occurring the expansion joint is incapable of accepting the pressure thrust. The hinge structure can be designed to accept the full pressure thrust when the pin is at the limit of its travel within the slot.
Gimbal Expansion Joints have a single bellows and a unique pressure thrust resisting structure which permits angular movements in any plane. The structure contains two hinge mechanisms, which allow simultaneous rotation about two perpendicular axes which intersect at the centerline of the bellows. This mechanism functions the same way that an automobile drive shaft universal joint operates.

Universal Expansion Joints consist of two bellows separated by a pipe spool so that large lateral movements can be accepted, in addition to axial compression and extension and angular deflections. These expansion joints have no restraints to resist pressure thrust and like the single, the piping designer must provide separate anchoring.

Tied Universal Expansion Joints contain two bellows separated by a pipe spool to accept large amounts of lateral movements. Because tie rods are provided to resist pressure thrust, these joints cannot accept axial movements. Also, if more than two tie rods are used, angular movements are not accepted. The tie rods are usually at or near ambient temperatures and therefore do not expand and contract as a function of the temperature of the media within the pipe. As a result, the thermal expansion of the length between the tie rods occurs within the expansion joint and is not applied to the rest of the piping system. The bellows design must accommodate this axial thermal expansion as well as the lateral movement for which the expansion joint is needed.

Hinged Universal Expansion Joints have two bellows separated by a pipe spool to accept large lateral movements. The ability to resist pressure thrust is provided by a hinge assembly at each bellows, which also permits angular movement in a single plane. Unlike the tied universal, the thermal expansion of the entire expansion joint is applied to the piping as if the joint was just another length of pipe.

Gimbaled Universal Expansion Joints are similar to the hinged universals above except that the two joints are gimbals type. The advantage of this arrangement is the ability of the expansion joint to accept large lateral movements in any plane at each end. Because the gimbals are attached each end of the bellows, the thermal expansion of the center spool will not be accepted by the universal, but must be accepted by the adjacent piping.

Swing Expansion Joints accept large lateral movements and angular movements in a single plane. They are similar to the hinged universal except that the individual hinge assemblies over each bellows are connected by swing bars rather than to the center pipe spool. Pressure thrust is accepted and the thermal expansion of the center pipe spool is kept within the expansion joint and is not applied to the connecting piping.

Pressure Balanced Expansion Joints are devices which produce no pressure thrust forces in the piping system on the main anchors. In addition to eliminating the pressure thrust, the expansion joint can accept axial compression, axial extension, lateral and angular movements. The balancing thrust is created by using a balancing bellows.

Pressure Balanced Elbows are expansion joints which can consist of a single or double bellows, but they also contain an elbow which is attached to its outer curvature, a balancing bellows and blind flange to the opposite end of the expansion joint and under pressure these tie rods balance the pressure thrust. This expansion joint can accept axial compression and extension and lateral movements. Special designs of the tying mechanism can permit some angular movements.

In-Line Pressure Balanced Expansion Joints consist of single or double (universal) bellows to accept the piping induced axial compression, extension, lateral and angular movements. The larger bellows create an angular pressure chamber that produces thrust forces, which, by tying to each end of the joint, balance the pressure thrust. They are typically used in straight pipe runs between intermediate anchors (non pressure thrust resistant) or adjacent to equipment, such as turbines, which cannot operate with heavy externally applied loads.


2.0 ACCESSORIES:

Liners can be installed inside the expansion joint to protect the elbows from damage when the following conditions exist:

 Smooth flow or low-pressure drop is required
 Velocities, which may produce flow induced vibrations described below.
 For air, steam and other gases
 Up to 6” dia. flow greater than 4 ft/sec per inch of dia. (up to 150 mm dia. flow greater than 0.05 M/sec per mm of dia.)
 Over 6” dia. flow greater than 7.5 M/sec)
 For water and other liquids
 Up to 6” dia. flow greater than 1.68 ft/sec per inch of dia. (up to 150 mm dia. flow greater than 0.02 M/sec per mm of dia.)
 Over 6” dia. flow greater than 10 ft/sec (over 150 mm dia. flow greater than 3.0 m/sec
 When high velocity, extremely turbulent or damaging two-phase flow exists upstream of the bellows, such as in the exhaust of a steam turbine.
 When extremely high temperatures are present, the liners can create an insulating barrier which would permit the bellows to operate at lower temperatures ensuring longer life and resisting oxidation. Steam purging and /or insulation can be added to enhance protection.
 When the media is erosive such as in catalyst carrying services.

When liners are specified, it shall be provided with sufficient diameter to allow the lateral movement expected to avoid interference.
Limit Rods are a safety device that protects the bellows from overextension due to anchor failure. These rods are passive unless a main anchor fails. During normal operation the rods have no function.

Covers should be specified when:

 Protection due to damage from foreign objects is needed, such as; falling tools or in high traffic areas.
 Protection of personnel is needed.
 Insulation will be applied over the expansion joint.
 When high flow velocities may exist around the outside of the expansion joint, such as in the exhaust of a steam turbine.
It is always safe to recommend a cover. The small cost addition can be insurance against premature failure and costly downtime due to damage. The standard cover is a removable design. The heavy covers are usually welded with full end closure rings for the maximum protection of the bellows, commonly used for turbine steam extraction lines.

Purge Connections are installed upstream of the bellows and downstream of the liner attachment to:

 Prevent packing or caking of media borne solids in the convolutions, which would prevent the bellows from free flexing.
 Introduce a cooling media such as steam between the outside of the liner and the inside of the bellows.

The standard purge connection shall be such that the purge media is introduced almost tangential to the circumference of the liner creating a cyclone effect thereby maximizing the purging action.

The number of purge connections around the circumference can be specified considering the size.

3.0 CONSIDERATION OF PARAMETERS TO SELECT EXPANSION JOINTS

 “Maximum Pressure” is the maximum pressure the bellows can accept as a design condition. Often bellows are capable of more than the normal pressure ratings because standard material thicknesses and manufacturing methods are used.
 “Maximum Movements” is the movement, which each EXPANSION JOINT CAN ACCEPT FOR 3000 CYCLES. This include the individual axial compression, lateral (shear or perpendicular to the pipe centerline) or angular maximum movements. Angular movement is limited to a maximum of 10 degrees in most cases to avoid potential pressure induced instability.
 The length and weights of the basic types of single expansion joints are other parameters. The bellows may be with a weld end or flanged on each end
 “Spring Rates”
o The axial spring rate is the force required to axially deflect the expansion joint one inch.
o The lateral or shear direction spring rates are also the force required to deflect the joint per inch.
o Moments from lateral spring rates are produced when lateral deflection occurs. This is the moment to hold the centerline of each end of the expansion joint parallel when the unit is deflected laterally by one inch.
o Angular spring rate is the moment required to bend the expansion joint by one degree.

 “Maximum Torque” is the torque which should not be applied to a bellows as excessive amounts may affect bellows cycle life. In some piping systems, elimination of this torque may not be practical. As part of a piping flexibility analysis, expansion joint bellows should be conservatively modeled with a tortional flexibility equal to that of the adjoining pipe. Bellow suppliers may be consulted if the resulting bellows tortional loading is of concern. It is usually possible to either redesign the bellows, or design hardware to relieve the bellows of this loading.

4.0 TIPS FOR SELECTING EXPANSION JOINTS

The first piece of information needed shall be the pipe size and the pressure and temperature of the piping system. All the expansion joint data available in suppliers catalogs are developed for 800 deg F (427 deg C).
Now, knowing the defections required, select the model with “Maximum Movements” values which exceed the requirements. Remember, each movement value is independent and is the full movement for 3000 cycles. This means that each value produces 100% of the stresses to yield 3000 cycles or that the axial value equals the lateral value in terms of bellows stresses. If the movements are in combination such as some axial and some lateral, the selected bellows must have each maximum movement exceed the required movements so that when the following formula is used the answer will be equal to or less than one (<1.0):

(Reqd. Axial/Max. Axial)+(Reqd. Lateral/Max. Lateral)+(Reqd. Angular/Max. Angular) < 1.0

If the system design requirements are for greater than 3000 cycles, the required movements can be adjusted mathematically so that a proper selection can be made. The following chart contains correction factors (Kc), which will convert your movements to 3000 cycle values which can then be used for selecting (see table below).

Cycles 100 500 1000 2000 3000 5000 7000 10000 20000 50000 100000 1000000
Kc 2.32 1.53 1.29 1.10 1.00 0.89 0.83 0.77 0.67 0.57 0.51 0.37

The above Kc values are interpolations from the EJMA fatigue curves. If your number of cycles falls between the values in the chart, select the Kc for the next higher number of cycles to yield a conservative result.

Find the Kc value for your number of cycles from the table. Multiply each of your movements by the Kc value. The new resulting values are the movements which can now be used to continue to make the proper selection from the expansion joint data sheets as previously described.

Caution: Extension movement more than 25% of the axial compression should be avoided.

Once the selection is made, the balance of the information on that unit can be noted, such as the length, weight and spring rates. If any of these values create a problem to the unit’s integration into the piping, the supplier should be notified so that a custom unit can be designed to fit.

Accessories such as internal lines, external protective covers and internal purge fittings can be added in the specifications.
5.0 Effects of Pressure Thrust
An example of the effect of pressure thrust is the comparison of a piece of pipe with blind flanges at each end, and a corrugated bellows with blind flanges at each end. When internal pressure is applied to the pipe, a pressure thrust force is exerted at the surface area (in2) of the blind flange. This force pushes the blind flanges in opposite directions. Since the pipe is rigid, the force is restrained by the pipe. However, corrugated bellows are flexible. When pressure is applied, the ends move in opposite directions, trying to un-corrugate the bellows.
In most cases, the force that is applied to the piping system is large when compared to the other forces in the system.

The pressure thrust (force) must be restrained by anchors, hardware (tie rods, hinges, etc.) or the force must be restrained by the equipment to which the expansion joint is attached. To determine the pressure thrust on an expansion joint, the following equation may be used;

Pressure Thrust (Lbs.) = Pressure (PSIG) X Effective Area (in2)

The effective area of the bellows can be found in the design data section of the suppliers catalogs.
6.0 Recommendations for Anchors and Guides
The proper anchoring and guiding of an expansion joint will control the motion of the expansion joint. This will insure that the joint is subject only to the deflection for which it was designed.
It is recommended that an expansion joint be located as near to an anchor as possible. The first guide should be located within four pipe diameters of the expansion joint. The second guide should be located within fourteen pipe diameters of the expansion joint. The remaining intermediate guides are placed at the approximate distances
Notes:
 An expansion joint that is placed in service should have a minimum of two guides. This is to avoid lateral offset or torsion that would occur if no guides are present.

 The magnitudes of pressure thrust and other forces can be very large. Therefore, U-bolts and spring supports should never be considered as proper guides.

The pressure thrust may also be restrained by tie rods, hinges, or gimbals if the major piping expansion is applied laterally to the expansion joint.